1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing pneumatic tyres.
2. Description of the Related Art
A pneumatic tyre for vehicle wheels generally comprises a carcass structure including at least one carcass ply having respectively opposite end flaps in engagement with respective annular anchoring structures, integrated into the regions usually identified as “beads”.
Associated with the carcass structure is a belt structure comprising one or more belt layers, disposed in radial superposed relationship with respect to each other and to the carcass ply and having textile or metallic reinforcing cords with a crossed orientation and/or substantially parallel to the circumferential extension direction of the tyre. Applied to the belt structure at a radially external position is a tread band also made of elastomeric material like other semifinished products constituting the tyre.
It is to be pointed out, to the aims of the present description, that by the term “elastomeric material” it is intended a composition comprising at least one elastomer polymer and at least one reinforcing filler. Preferably, this composition further comprises additives such as cross-linking agents and/or plasticizers, for example. Due to the presence of the cross-linking agents, this material can be cross-linked through heating so as to form the final manufactured article.
Respective sidewalls of elastomeric material are also applied to the side surfaces of the carcass structure, each extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads. In tyres of the tubeless type, an airtight coating layer, usually called “liner” covers the inner surfaces of the tyre.
Generally, in the manufacture of tyres for vehicle wheels it is provided that subsequently to building of the green tyre through assembly of the respective components, a moulding and curing treatment be carried out which aims at determining the structural stabilisation of the tyre through cross-linking of the elastomeric compositions and also at forming a desired tread pattern thereon, as well as possible graphic distinctive marks at the tyre sidewalls.
Manufacturing processes of recent conception are known in which building of the green tyre is carried out by making the different components thereof on a forming surface externally shaped as a toroidal support, the conformation of which is coincident with the inner conformation of the finished tyre. For carrying out the moulding and curing treatment, the green tyre is enclosed in the moulding cavity of a vulcanisation mould of a shape matching the outer conformation to be given to the finished tyre, together with the toroidal support on which the tyre itself has been built.
WO 01/00395, in the name of the same Applicant, depicts a method of the above mentioned type involving use of a toroidal support the forming surface of which has a substantially U-shaped outline in a diametral section plane of the toroidal support.
During closure of the vulcanisation mould, circumferential seats defined in the mould itself interfere against the radially internal edges of the shaped wall causing such an elastic deformation of the toroidal support that a high contact pressure is ensured between the circumferential seats of the mould and the radially internal edges of the shaped wall.
The toroidal support has an outer diameter slightly smaller than the inner diameter of the finished tyre. The tyre crown region is moulded against the inner surface of the moulding cavity following a radial expansion induced by steam under pressure introduced into a diffusion interspace defined between the toroidal support and the inner surface of the tyre itself.
WO-2004/045837 in the name of the same Applicant as well, proposes that preliminary heat be supplied to the tyre through feeding of a fluid in counter-pressure into the mould externally of the tyre, to counteract the pressure of steam or other fluid fed to the inside of the toroidal support. Thus heat can be supplied to a temperature high enough and for a time sufficiently long to ensure a correct consolidation of the beads an a sufficient cross-linking of the liner before carrying out moulding of the tyre crown portion through feeding of high-pressure steam into the diffusion interspace.
On practically carrying out said processes the Applicant could note different difficulties correlated with the tyre moulding and curing treatment.
In a moulding and vulcanisation process for tyres as the one disclosed in WO-01/00395, and/or in WO-2004/045837 an “imposed-volume” moulding is required on wide tyre regions extending, by way of indication, from the beads along the sidewalls and to the shoulders.